Discovery of spin-up in the X-ray pulsar companion of the hot subdwarf HD 49798

• Published in: Monthly notices of the Royal Astronomical Society 2016-06, Vol.458 (4), p.3523-3527
• Main Authors: Mereghetti, Sandro, Pintore, Fabio, Esposito, Paolo, La Palombara, Nicola, Tiengo, Andrea, Israel, Gian Luca, Stella, Luigi
• Format: Article
• Language: English
• Subjects: Accretion; Accretion disks; Dwarf stars; Neutron stars; Pulsars; Spectrum analysis; Star & galaxy formation; Stellar winds; Subdwarf stars; White dwarf stars; X-ray astronomy; X-rays; XMM (spacecraft)
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stw536

The hot subdwarf HD 49798 has an X-ray emitting compact companion with a spin-period of 13.2 s and a dynamically measured mass of 1.28 ± 0.05 M⊙, consistent with either a neutron star or a white dwarf. Using all the available XMM–Newton and Swift observations of this source, we could perform a phase-connected timing analysis extending back to the ROSAT data obtained in 1992. We found that the pulsar is spinning up at a rate of (2.15 ± 0.05) × 10−15 s s−1. This result is best interpreted in terms of a neutron star accreting from the wind of its subdwarf companion, although the remarkably steady period derivative over more than 20 yr is unusual in wind-accreting neutron stars. The possibility that the compact object is a massive white dwarf accreting through a disc cannot be excluded, but it requires a larger distance and/or properties of the stellar wind of HD 49798 different from those derived from the modelling of its optical/UV spectra.